The present invention relates in general to filtration modules or elements, and in particular to a new and useful method and apparatus which utilizes a curved or coiled tubular membrane constructed to maximize the formation of Dean vortices at the solution-membrane interface, for improving the filtration effect.
Most modular designs for pressure-driven membrane processes, such as reverse osmosis, ultrafiltration and microfiltration are based on maximizing membrane area per unit volume and on the handling convenience of the module. Many methods exist for reducing CP and fouling, including chemical modification of the membrane surface and physical methods such as scouring. Hydrodynamic methods are also known which rely on eddies during turbulent flow, or induced flow instabilities. Such instabilities can be created by introducing inserts into the flow path. Unstable flow across membranes have also been utilized to reduce solute build-up at the solution-membrane interface, by the inventor of the present application. See Belfort, G., "Fluid mechanics in membrane filtration: recent developments", J. Membrane Sci., 40, 123-147 (1989).
Different types of instabilities have been used including vortices and instabilities resulting from rough membrane surfaces, flow pulsations and oscillating membrane surfaces. In addition to rough membrane surface, etc., instabilities have also been induced by a rotating disc system developed (i) in the 1970's by Fred Littman and Jerry Croopnick at Dresser Industries, TX and before that at Stanford Research Laboratories, CA and (ii) in the 1990's by Brown Boveri Co. in Malmo, Sweden. One of the most successful depolarizing methods has used Taylor vortices established in a rotating annular filter module. The main limitations of this design are the difficultly in scaling-up membrane area and high energy consumption. Vortices have also been produced in membrane-lined channels by frequently reversing turbulent flow (at 8 Hz) in a corrugated channel. See, Stairmand, J. W. and Bellhouse, B. J., "Mass transfer in a pulsating turbulent flow with deposition into furrowed walls," Int. Heat Mass Transfer, 27, 1405 (1985). This has also been done by forcing the fluid to flow around in a spiral half-cylinder channel over a flat membrane. See PCT patent application WO 90/09229 of Aug. 23, 1990 to Winzeler. Both of these approaches show increased performance in the presence of vortices, but each has experienced some difficulties including scale-up and sealing problems. The use of spacers in a spiral wound unit to induce mixing has been widely recommended. Unfortunately, at low axial Reynolds numbers typically used in Poiseuille flow for most spiral wound units, this approach has been largely ineffective. See the spiral flow filters of Toray Industries, Inc., disclosed, for example, in their brochure entitled Romembra Toray Reverse Osmosis Elements.
The effective use of fluid instabilities, such as vortices, in depolarizing and cleaning synthetic membranes for pressure-driven membrane applications has been widely confirmed in the literature. See Winzeler, H. B. and Belfort, G. (1993), Enhanced performance for pressure-driven membrane processes: The argument for fluid instabilities, J. Membrane Sci., in 80, 35-47. The present invention has shown excellent flux improvements in the presence of Dean vortices resulting from flow around a curved duct with microfiltration membranes. See U.S. Pat. No. 5,204,002, which is incorporated here by reference. An object of the present invention is to provide for such controlled vortices to be used to depolarize salt, macromolecules and suspensions in high pressure reverse osmosis (RO), ultrafiltration (UF), microfiltration (MF) or nanofiltration (NF) membrane processes.